Sequencing research is reevaluated using prospective data from young cohorts and is extended by broadening the concept of sequencing to consider when regular use and problem use occur and when use of more serious drugs begins. Two data sets from studies of alcohol and drug use among youth and young adults conducted by the Research Triangle will be analyzed in this expanded research. The data sets cover the time period between 6th grade and age 27. The major objective of the proposed research is to extend current knowledge about the development and maintenance of substance use patterns by broadening the concept of the sequencing of substance use and by considering the determinants of transitions between nonuse, initiation, regular use, problem use, and cessation. Five specific aims will guide analyses to accomplish this major objective. 1. Examine sequencing of initiation of alcohol, drugs, and tobacco for all youth and subgroups of youth defined by sex, race/ethnicity and urban/rural status; 2. Examine sequencing and timing of initiation, regular use, and problem use of alcohol, drugs and tobacco for all youth and subgroups of youth defined by sex, race/ethnicity and urban/rural status; 3. Examine the extent to which alcohol, drug, and cigarette use are maintained once initiated, whether youth use multiple drugs concurrently or stop using "lesser' drugs once they start using more serious drugs. 4. Examine the relationship of age of initiation and more regular use to problem use and negative consequences of specific substances in youth and young adulthood to determine the causal importance of age of initiation versus regular use and early use versus later use; and 5. Examine the risk factors for progression in substance use by examining the predictors of transitions from nonuse to initiation, from initiation to regular use, and from use to cessation of specific substances and across substances and across substances, including risk factors such as peer, school, familial, and personality factors; substance use availability; and sociodemographic characteristics. These analyses will broaden current understanding of the development of substance use patterns by considering the sequencing and timing of regular and problem substance use as well as the initiation of substance use and the importance of age of initiation relative to age of regular use in predicting problem use and negative consequences. This broader approach will place sequencing research on substance use within a broader context that recognizes the importance of transitions to more frequent or problem use.